The overall objective of the proposed research is to study the physiology of the mammalian kidney, in particular that of the renal medulla. The experiments described concern three general problems: the urinary concentrating mechanism, intrarenal heterogeneity in nephron function, and calcium reabsorption by the renal tubule. The approach to each problem will be direct, using micropuncture and related microanalytical techniques, and will involve both normal rats, parathyroidectomized rats, and a strain of rats with hereditary diabetes insipidus. Specific problems to be studied during the next year include: Uninary concentrating mechanism. We plan to study the effect on the flow and composition of fluid in the loop of Henle, vasa recta and collecting tubules of the acute intravenous infusion of urea to protein rats. Where others have shown this to increase maximum urinary osmolality substantially. Calcium. We plan to continue our study of the relationship between calcium and sodium reabsorption in the loop of Henle and in the superficial nephron.